A fault tolerant queue system for selecting storing positions in a memory for performing write operations from an on-line maintained idle list of addresses to free storing positions in the memory and for calculating addresses for collecting data earlier stored in the memory. The storing positions are...http://www.google.com/patents/US6088817?utm_source=gb-gplus-sharePatent US6088817 - Fault tolerant queue system

A fault tolerant queue system for selecting storing positions in a memory for performing write operations from an on-line maintained idle list of addresses to free storing positions in the memory and for calculating addresses for collecting data earlier stored in the memory. The storing positions are exposed to a testing procedure. The testing procedure is performed periodically on each storing position in idle cycles during operation of the system, and the result is used to maintain the idle list. The addresses selected for testing are released from system use as controlled by the system.

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Claims(14)

What is claimed is:

1. A memory management system for selection of storage locations in a memory for performing write operations on these from an on-line maintained idle list of addresses to free storage locations in the memory, and calculation of addresses for collection data earlier stored in the memory,

wherein the storage locations are exposed to a testing procedure having two modes of operation including:

a first mode in which all operation of all storage locations is tested in an off-line test, controlled by test logic having exclusive control over the entire memory, and

a second mode on-line during which the memory testing logic has exclusive control over one storage location at a time, said storage location being exposed to writing of a test pattern and subsequent read back of said test pattern only after all other storage locations, not barred from use, have been written to by other processes in the system or by the memory testing procedure,

the testing procedure being performed periodically on each storage location in idle memory cycles during performance of normal operation of the system, and the result of which is used for maintenance of the idle list, the addresses selected for test being released from system use as controlled by the system.

2. A system according to claim 1, wherein the test of a storage location is performed after awaitance of it having been released from use by the system, and return to system use is performed only if the storage location stands the test.

3. A system according to claim 1, wherein addresses to the storage locations are released by copying the contents of the storage location to be tested to a free and error free storage position, whereupon the address calculation is redirected for use by the new storage location, and finally the tested storage location is returned to system use only if it stands the test.

4. A system according to claim 1, wherein storage locations having not standed a test are retested at occasions when an entire memory block containing the storage location in question is released from use in the system.

5. A system according to claim 1, wherein the memory testing logic for performing the testing of a determined storage location operates

in the first mode in which all addresses are tested in an off-line test controlled by the testing logic, during which counting is performed of the number of storage locations with correctable errors and in which existing not correctable errors are indicated, and

in the second mode on-line, during which the memory testing logic only checks data to the storage location being tested.

6. In a memory management system for selection of storage locations in a memory for performing write operations on these from an on-line maintained idle list of addresses to free storage locations in the memory, and calculation of addresses for collection data earlier stored in the memory, a method for testing said storage locations, comprising

releasing addresses selected for test from system use as controlled by the system,

exposing selected storage locations to a testing procedure periodically on each storage location in idle memory cycles during performance of a normal operation of the system, wherein the testing procedure comprises testing, in a first mode all operation of all storage locations off-line, controlled by test logic having exclusive control over the entire memory and testing, in a second mode on-line during which the memory testing logic has exclusive control over one storage station at a time, said storage location being exposed to writing of a test pattern and subsequent read back of said test pattern only after all other storage locations, not barred from use, have been written to by other processes in the system or by the memory testing procedure, and

using the results of said testing procedure for maintenance of said idle list.

7. A method according to claim 6, comprising

performing the test of a storage location after awaitance of it having been released from use by the system, and

returning the storage location to system use only if the storage location stands the test.

8. A method according to claim 6, comprising

releasing addresses to storage locations by copying the contents of a storage location to be tested to a free and error free storage location,

redirecting the address calculation for use by the new storage location, and

returning the tested storage location to system use only if it stands the test.

9. A method according to claim 6, comprising

retesting storage locations having not standed a test at occasions when an entire memory block containing the storage location in question is released from use in the system.

10. A method of claim 6, comprising

a) performing testing of a determined storage location by

testing in the first operational mode all addresses in an off-line test controlled by the testing logic,

counting during said testing the number of storage locations having correctable errors and

indicating not correctable errors,

b) checking in the second mode on-line only data to the storage location being tested.

11. A memory addressing system for selection of storage locations in a memory for performing write operations on these from an on-line maintained idle list of addresses to free storage locations in the memory, and calculation of addresses for collecting data earlier stored in memory,

wherein the storage locations are exposed to a test procedure, executing continuously and repeatedly, for cyclically testing and retesting all storage locations one at a time,

the test of a storage location is performed after awaiting its release from use by the system, and return to system use is allowed only if the storage location stands the test, the result of the test procedure being used for maintenance of the idle list, such that any storage location becomes available for use by other system tasks only if it stands the test, the addresses selected for test being removed from use by other system tasks as controlled by the system,

memory testing logic for performing the testing of a determined storage location operates in a main on-line mode while the system is in full operation, during which the memory test logic controls data in storage locations not used by other system tasks, while data in all storage locations in use by the system is under control by the system tasks,

the test logic selects each storage location for a test procedure after having made sure that the selected storage location is released from use by other system tasks,

the selected storage location is allocated for exclusive use by the test procedure,

the test logic exposes the selected storage location to first and second tests following on each other, each beginning with writing a first test word and a second test word, respectively, into the selected location, and

each of the first and second tests then proceeds by a procedure in which the test logic in turn sends a signal requesting start of a timer to an addressing logic, and waits for a timer signal in return informing that all storage locations, that are not error marked, have been written into at least once after start of the timer,

writes, while the timer is running, the other test word, not having been written into the selected location, into idle error free storage locations other than the selected storage location using a Write instruction followed by a Read-release for each location, or a Write-Background instruction if available, which immediately releases each such storage location for use by other tasks,

waits for a timer signal, informing that all storage locations have been written, from the addressing logic and when received, issues a Read-Keep operation on the storage location selected for test, for verifying that its content is still the test word written into the selected storage location at start of the current one of the first or second tests, and

releases, if both of the first and second tests on the selected storage location pass, the selected storage location for use by other system tasks, and otherwise considers it erroneous and blocks it out from system use.

12. A system according to claim 11, wherein addresses to the storage locations are released by copying the contents of the storage location to be tested to a free and error free storage location, whereupon the address calculation for other system tasks is redirected to the new storage location.

13. A system according to claim 11, wherein storage locations having once failed a test are retested at occasions when the whole memory block containing the storage location in question is released from use in the system.

14. A system according to claim 11, wherein the test logic also operates in an off-line mode targeted for manufacturing test in which data in all addresses is controlled by the test logic, during which counting is performed of the number of storage locations with faults that can be overcome by coding data into a given error correcting code format and in which any storage location with a fault that can not be overcome by coding into the given error correcting code, is indicated.

Description

This application is a continuation, of application Ser. No. 08/612,187, filed Mar. 7, 1996, which is a divisional of application Ser. No. 08/339,672 filed Nov. 14, 1994, now U.S. Pat. No. 5,602,988.

BACKGROUND

The present invention according to a first aspect relates to a queue system for selection of storing positions in a memory for performing write operations on these from an online maintained idle list of addresses to free storing positions in the memory. The queue system also calculates addresses for collecting data earlier stored in the memory.

In accordance with second and third aspects the invention relates to a queue system for buffering data in a packet switch with a common buffer memory for one or more ports of the switch. The system uses a number of pointers identifying storing positions in the buffer memory. The pointers are moved between different logical lists for indicating the operation to be performed on the data in the buffer position pointed to.

The invention furthermore relates to methods for testing storing positions and pointers, respectively, in connection with queue systems of the kind indicated above.

When memories are used in computers and other equipment, address calculation decides the physical positions to be used for storing of a certain data. Most simply direct addressing, decision or calculations of which address to be used is performed at compiling of a program, or in connection with hardware design for devices having fixed programs. Indirect addressing and linked lists are examples of more flexible addressing methods. Here the address information is stored in other address positions or in counters, etc. The address calculation is thus not fixed but varies during execution for adaption to amount and format of the stored data.

Simple FIFO memories with direct addressing most often are realized with counters for incrementing write and read addresses, or in their most simple form are realized with fixed addresses in shift registers. In connection with dynamic indirect addressing address pointers are used, which are also stored in memories. At system start these pointers are initialized so as to obtain a valid set of pointers for the system to work with. The function of the system is then dependent on this pointer initialization being not corrupted during operation. Bit errors in a pointer value results in a lasting malfunction in the system. For systems being in continous operation with high demands on reliability this is a problem.

Great semiconductor memories are traditionally equipped with redundance for improving the yield of the manufacture. By redundancy is here meant rows and columns of spare circuits, which may be connected into circuit to replace ordinary circuits. This is obtained by fusing fuses or by other means for permanently reconfiguring the circuit in the factory.

Testing is performed for determining circuits requiring reparation. If manufacturing errors occur and are such that they can be repaired by connection into circuit of spare circuits, the type of manufacturing error is determined and the repair is performed.

For semiconductor memories without redundancy, mapping between address and physical position in the memory is determined while designing the memory. For semiconductor memories with redundancy this mapping is partly set during the design and is completed during manufacture when erroneous memory positions shall be repaired.

In the case of disc memories a testing procedure is used before taking the disc into operation, during a formatting operation for determining the sections on the disc which may be used. The aim is to be able to use a disc with bad sections. The error hypothesis used is that certain positions on the disc may be bad due to the manufacture being not perfect. The dividing in good and bad sectors may be performed during the formatting process and is assumed not to need to be changed. The risk for disc addressing errors need not be taken into consideration during the formatting procedure.

In certain systems error correcting codes (ECC=Error Correcting Codes) are used for obtaining error tolerance against bit errors during operation. These errors occurring during operation may be soft or hard, i.e. temporary disturbances or fixed errors. The number of bits able to be corrected in each word is delimited. The code most often used may correct single bit errors and detect double bit errors. Quite recently it has been shown that a combination of error correcting codes and spare rows provides an error tolerance system which has a very high degree repairability of manufacturing errors in semiconductor memories, cf. IEEE transactions on computers, vol. 41 #9 September 1992, pp 1078-1087.

Repair of devices before they leave manufacture requires testing apparatus and equipment for fuse programming (a lasting change of connections in an electric circuit for influencing its function--also called fuse or antifuse), on the manufacturing line in the factory, and a manufacturing process including fuse components. This implies increased costs due to a prolongation of the manufacturing cycle. The degree of success with respect to the repair operations is limited by the fact that they are based on the use of spare rows and columns, and by the inflexibility with respect to the handling of sporadic single errors.

Use of error correcting codes takes care of isolated single errors. For full efficiency there is, however, required a combination with some sort of system with redundant rows. According to the state of the art this implies fuse programming and testing before the circuits leave the manufactory.

U.S. Pat. No. 4,049,956 provides an example of the state of the art as regards error detection during operation and describes supervision of a memory working in a time multiplexing mode. Incoming data words are stored temporarily in the respective steps of a multiple step memory and are read out for further revision. The testing is performed without taking the memory out of normal operation. The memory consists of a number of n steps which are periodically addressed in a repeated scanning cycle of n time slots in which binary words are written into and read out from the respective steps. Words written into a memory step are read out in the immediately following cycle. Malfunctions are detected by means of parity checks.

U.S. Pat. No. 3,863,227 describes the selection and testing of each one of individual electronic control elements used for accessing the different core elements in a core memory when the memory is on-line. There is thus the question of on-line test of a memory system, but not of the memory storing elements included in the memory. No use of parity tests is described.

U.S. Pat. No. 5,016,248 describes a queueing system for packet transfer without mentioning initialization or maintenance.

SUMMARY

A first object of the present invention is to provide error tolerance by a continous maintenance of the pointer handling in a queue system with dynamic indirect addressing so that the system always has the ability of resetting a correct set of pointers independently of the state in which the system has ended.

A second object is to attain error tolerance against errors in memories which can occur during operation of the system.

A third object of the invention is to improve the yield at manufacture of great semiconductor memories by organizing and using redundant circuit in a new way. By using error tolerance, memories with small manufacturing errors may operate in a correct manner and need not be rejected. By suitable setting of the rejection level during the testing procedure so as to maintain spare capacity, the memory handling system may also recover from errors occurring during operation.

According to the invention the above mentioned objects are obtained entirely or partly in the following way.

In a queue system according to the first aspect indicated by way of introduction the storing positions are exposed to a testing procedure which is performed periodically on each storing position in idle cycles during operation of the system. The result of the procedure is used for maintenance of the idle list. The addresses selected for test are released from system use as controlled by the system.

In one embodiment the test of a storing position is performed after awaitance of it having been released from use by the system, and return to system use is performed only if the storing position stands the test.

In another embodiment addresses to the storing positions are released by copying the contents of the storing position to be tested to a free and error free storing position. Thereupon the address calculation is redirected for use by the new storing position. Finally the tested storing position is returned to system use only if it stands the test.

In a further embodiment storing positions having not standed the test are retested at occasions when the whole memory block containing the storing position in question is released from use in the system.

In still a further embodiment memory testing logic for performing the testing of a determined storing position operates in a first mode in which all addresses are tested in an off-line test controlled by the testing logic, during which counting is performed of the number of storing positions with correctable errors and in which existing not correctable errors are indicated, and in a second mode on-line, during which the memory testing logic only checks data to the memory position being tested, and other storing positions are brought to operate with data during normal operation as address controlled by the system.

In a queue system according to the second aspect indicated by way of introduction a maintenance function continously secures that the system includes one and only one pointer to each valid packet position in the buffer memory.

In a queue system according to the third aspect indicated by way of introduction copies of the same pointer are handled in a controlled way by using a multiple pointer list in which the number of copies included in the system of respective pointers are recorded, and in which a maintenance function checks that the number of copies of a pointer agrees with the value recorded in the multiple pointer list for the respective pointer.

In a first embodiment the following logical lists for the respective pointer categories are used besides the multiple pointer list: an idle list for idle pointers, output queue lists for queued pointers in a queue to output ports, and a blocking list for blocked pointers. The maintenance function cyclically checks all pointers and appurtenant buffer positions, one at the time, preceded by an initiation procedure. During this procedure the pointer is taken out of operation by filtering out the pointer from the flow of pointers, which from the output queue lists are returned to the idle list until the queue system has been emptied of each copy of the pointer in question, whereupon the number of returned copies of the pointer in question is compared with the value in the multiple pointer list.

In a first further development of this embodiment the maintenance function has a first operational mode which, after the initiation procedure, and activated by either the fact that more than a determined limit number of pointers have been found as a result of said initiation procedure, the respective number of copies of which does not agree with the value in the multiple pointer list for the pointer in question, or by an external signal, returns the pointers to the idle list, resets the value in the multiple pointer list to no multiple copies, and in the blocking list indicates no blocking of pointers.

In a second further development the maintenance function has a first operational mode which, after the initiation procedure, and activated by the fact that either more than a determined limit number of lost or multiple pointers have been found as a result of said initiation procedure, or by an external signal, returns a copy of each pointer to the idle list, and in the blocking list indicates no blocking of pointers.

The maintenance function may have only one further operational mode, i.e. a second mode. In that case this second operational mode is activated by the first maintenance mode having been run through for all pointers. Thereupon the initiation procedure is again performed on each pointer, one at a time, and a buffer memory test is queued with low priority on non-blocked memory positions identified by the pointer in question. All other non-blocked memory positions are exercised with system data during the test. The result of the memory test is used for returning the pointer to the idle list if the buffer memory test has given a correct result, but it is blocked from use in the system if the buffer memory test indicates malfunction of the area defined by the pointer. The blocked pointers are exposed to a second buffer memory testing of the same type as the first one conditioned by each testing data operation is maintained in the queue as long as system data is stored anywhere in the buffer memory segment including the positions which have been identified by blocked pointers. The result of the second test is used for returning the pointer to the idle list if the test has given a correct result, whereas blocked pointers are returned to the idle list first after having been accepted in a number of consecutive tests exceeding a second limit number.

In case the maintenance function has a number of different operational modes, it may have a second operational mode activated by the first mode having been run through for all pointers, whereupon the initiation procedure is again performed on the pointers. A simple write and read test is queued for execution on buffer memory positions identified by pointers the number of copies of which does not agree with the value in the multiple pointer list. The result of the write and read test is used for returning the pointer in question to the idle list if the test has given a correct result, but blocking it out from use in the system if the test indicates malfunction within the area identified by the pointer in question.

A third operational mode is activated by the second mode having been run through for all pointers, whereupon the initiation procedure is once again executed on the non-blocked pointers. A buffer memory test for detecting types of malfunctions having not been detected by the simple write and read test in the second operational mode is queued for execution on the buffer memory positions which have been identified by the initiated pointer, said memory positions being exercised with testing data, whereas all other, non-blocked buffer memory positions are exercised with system data during the test for each blocked pointer. The result of the test is used for returning the pointer to the idle list if the buffer memory test has given a correct result, but blocking it out from use in the system if the buffer memory test indicates malfunction in the area identified by the pointer.

A fourth operational mode is activated by the third mode having been run through for all pointers, whereupon also the earlier blocked pointers are accepted for testing by performing said initiation procedure cyclically for all pointers, one at a time. A buffer memory test of the same kind as in the third mode is applied on non-blocked pointers whereas earlier blocked pointers are exposed to the same type of buffer memory testing conditioned by each testing data operation being maintained in queue as long as system data is stored anywhere in the buffer memory segment containing the positions having been identified by the earlier blocked pointer in question. The result of the testing is used in the same way as in the third mode for non-blocked pointers and non accepted tests of earlier blocked pointers, whereas earlier blocked pointers are returned to the idle list first after having been accepted in a number of consecutive tests defined by a limit number.

The memory handling system according to the invention provides a dynamic tolerance against manufacturing errors when the system is in operation. Testing and repair by reconfiguration in the factory is not required. Testing is performed after completed manufacture for determining whether the memory has storing capacity enough or not for the intended application.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described more closely below with reference to an embodiment shown on the attached drawing, in which

FIG. 1 very schematically illustrates a queue system according to one of the aspects of the invention,

FIG. 2 partly in block form illustrates an embodiment of a packet switch, in which the different aspects of the invention are used,

FIG. 3 shows a flow diagram of main operation modes included in a maintenance function for testing address pointers and buffer memory positions in the packet switch according to FIG. 2, and

FIGS. 4-7 show flow diagrams for the respective modes of FIG. 3.

DETAILED DESCRIPTION

FIG. 1 very schematically illustrates a queue system for selecting storing positions in a data memory 2 for performing write operations, arrow 4, on these from an on-line maintained idle list 6 of addresses to free storing positions in the memory 2, and calculation of addresses for collecting, arrow 8, data earlier stored in the memory 2. The last mentioned addresses are transferred, arrow 10, to lists 12 over used storing space in the memory 2.

According to a first aspect of the invention the memory positions of the memory by means of a testing logic 14 are subjected to a testing procedure which is performed periodically, arrow 16, on each storing position during idle cycles during operation of the system. The positions to be tested are selected, arrow 18, by means of the addresses included in the lists 12, the selected positions being released from system use controlled by the system. The result of the test is used for maintenance, arrow 20, of the idle list 6.

The testing of a storing position may be performed after awaitance of it being released from system use. Alternatively addresses to storing positions to be tested may be released by copying the contents of such a storing position to be tested into a free and error free new storing position. The address calculation is thereupon redirected for use of this new storing position.

In both cases a tested storing position is finally returned to system use, i.e. to the idle list 6, only if it can stand the testing. Storing positions not standing the testing are retested at occasions when all the memory block including the storing position in question is released from use in the system.

The memory testing logic 14 for performing the testing of a given storage position may preferably operate in two modes.

In the first mode all addresses are tested in an off-line test controlled by the logic, during which counting is performed of the number of storing positions with correctable errors, and in which existing not correctable errors are indicated.

During this component testing a complete memory test is performed with e.g. so-called "march patterns", cf. e.g. "Using March Tests to Test SRAMS", AD J van de Goor, IEEE Design & Test of Computers, March 93, vol 10, no 1, and "Built in Self Diagnosis for Repairable Embedded RAM:S, Robert Trenter, et al, IEEE Design & Tests of Computers, June 93, vol 10, no 2. This is done for eliminating not operable circuits. Since the queue system according to the invention offers a certain amount of error tolerance, also circuits with erroneous storing positions may be classed as correct. Therefore the number of errors are recorded during the testing. If the number of errors is lower than the set rejection threshold the component is accepted.

In the second mode, acting on-line, the logic 20 only supervises data to the memory position being tested, and other storing positions are forced to operate with data during normal operation as address controlled by the system.

An embodiment of the invention will now be described more in detail. In one type of packet switch, and with reference to FIG. 2, an arriving packet is written, after analysis of the contents in the head of the packet in an analysing function 30, into a storing position in a buffer memory 32, common to the arriving packets. The buffer memory 32 is shown schematically with an arriving packet flow 34 and a number of output channels 36.1 . . . 36.n for packet flows leaving the switch. The flow of data to the analysis function 30 for enabling this function has been designated 34'.

The writing is performed guided by an address list 38 containing address pointers pointing to corresponding idle storing positions in the buffer memory 32. This address list is also shortly called "idle list" below. More particularly, the analysis function 30 with a signal, according to arrow 39, instructs the idle list 38, on the one hand, to provide a pointer for the packet in question and, on the other hand, after writing, signalled according to arrow 40 from the idle list 38 to the buffer memory 32, of the packet into the buffer memory guided by the pointer, to transfer the pointer to a pointer queue system containing a number of FIFO pointer queues 44, one for each output 36.

More particularly, the pointer is thus moved according to arrow 42 from the idle list 38 to a FIFO pointer queue 44.n for a switch output 36.n which has been selected for the packet guided by the analysis of the contents of its head. The selection of pointer queue is performed by a control signal 45 from the analysis function 30 to the pointer queue in question.

The head of an arriving packet may contain several destination addresses, i.e. the contents of the packet may be sent to several switch outputs. In this case the pointer is copied to the selected storing position in the buffer memory 32 so as to obtain as many pointers as the number of switch outputs in question. Below also the denomination "multiple pointer" is used for a pointer in such a set of pointers which points to the same storing position in the buffer memory 32. These multiple pointers are guided to each one of the pointer queues for the outputs in question.

When a pointer, i.e. a single pointer or a multiple pointer, has appeared as the first one in its FIFO queue 44.n, the data packet pointed to by the pointer is read, according to arrow 46.n and guided by the pointer, from its storing position in the buffer memory 32 to the output 36.n in question.

Thereafter the pointer is redirected according to arrows 48 and 50 to the idle list 38 via a maintenance function 52, to be described more closely below, which supervises pointers to be returned to the idle list 38. More particularly, the pointer is moved to the idle list if it is not selected for testing by the maintenance function 52.

In the maintenance function 52 copies of the same pointer are handled in a controlled way. More particularly, this is performed by using a multiple pointer list 56 included in the maintenance function 52 in which the number of copies of the respective pointer included in the system are recorded. The maintenance function 52 also checks that the number of copies of a pointer agrees with the value recorded in the multiple pointer list 56 for the respective pointer.

In a more detailed description below of embodiments of the invention the denomination x is used for a pointer being tested, where x belongs to the set of all pointers. Correspondingly, the buffer memory position in the buffer memory 32 pointed to by the pointer x is designated B(x).

The maintenance function 52 in a way to be described more closely below, performs testing of buffer memory positions B(x). For this purpose the pointer x is transferred to a testing logic 58 for the buffer memory 32 according to the arrow 60'. The result of the test operation on the buffer memory position B(x) is signalled from the test logic 58 to the maintenance funtion 52 according to the arrow 60".

It will likewise appear more closely from the following description that if this testing indicates malfunction of a memory position the corresponding pointer will be blocked from use in the system and is supplied to a blocking list 62 likewise included in the maintenance funtion 52. The lists for multiple and blocked pointers may also be combined to a single list.

More particularly, the maintenance function 52 cyclically checks all pointers and appurtenant buffer positions, one at a time, preceded by an initiation procedure. With "all" pointers is here meant all pointers which shall exist in the system, i.e. pointers included in the idle list 38, in the output queues 44, in the multiple pointer list 56 and in the blocking list 62. During the initiation procedure each pointer value, one at a time, is taken out of operation for checking, on the one hand, that the handling of pointers of the queue system 38, 44 is correct, and, on the other hand, for checking the data storing area B(x) in the buffer memory 32, identified by the pointer in question.

During the initiation procedure the queue system 38, 44 is emptied of the pointer under test and of all copies thereof, if any, whereupon a check is made that the number of found copies is equal to the value in the multiple pointer list 56 for the pointer under test.

The maintenance function 52 can thereafter operate according to one or several operating modes. Irrespective of the alternative prevailing, the initiation procedure is the first step of each such operational mode.

With reference to FIG. 3 and in one embodiment of the case that there are several operational modes, there may be four levels 70, 72, 74 and 76 of the maintenance function 52. The lowest level 70 is denominated "Pointer conditioning" and is only directed to the circulation of pointers. The other three ones are extended with different read/write tests of the buffer memory 32.

As has been mentioned, the queue system 38, 44 is emptied of all copies, if any, of the pointer x during the initiation procedure. In non real-time systems this may be obtained in several different ways. In real-time systems one is usually reduced to working only with pointers being returned to the idle list 38.

For emptying a real-time queue system of the kind described above of the pointer x, the idle list 38 must first be emptied of all copies, if any, thereof. This is performed by filtering out the pointer x from the flow 48 of returned pointers, step 78 in FIGS. 4-7, so as to prevent it from being written into the idle list 38. More particularly this filtering is enabled in that the maintenance function 52 in accordance with the arrow 78' emits a signal to the idle list 38 to put an indicator for the pointer being the last one in the idle list for the moment. By means of a signal 78" the maintenance function 52 thereafter obtains information that the indicator has reached the most forward position in the idle list 38. When all pointers that existed in the idle list 38 at the beginning of the filtering step have been fed to output queues 44, the idle list 38 is emptied of all pointers x for certain.

Thereafter the output queues 44 must be emptied of copies, if any, of the pointer x. This emptying is enabled, more particularly, by the maintenance function 52 emitting, according to the arrow 79', a signal to the output queues 44 to set an indicator for the pointer being last in the respective output queue for the time being. With a signal according to the arrow 79" the maintenance funtion 52 thereafter obtains information from the output queues that the pointers thus indicated have ended first in the respective queue. The maintenance function 52 thus awaits, while continously filtering out pointers x from the flow 48 of returned pointers, that the pointers which were located last in the output queues 44 at the time when the emptying of the idle list 38 had ended, shall end first in the queue. When the slowest queue has fulfilled this criterion, the system is emptied of the pointer x.

Thereafter, in step 80 in FIGS. 5-7, a check is made with respect to whether the number of returned pointers agrees with the value for the pointer x in the multiple pointer list 56. If this is not the case a pointer error signal is generated. This error signal ends in somewhat different measures dependending on the maintenance mode being executed. In all modes except the mode "Pointer conditioning" the pointer value is blocked and an error counter, not shown, is incremented, step 82 in FIGS. 5-7. If more than a number of a errors are found, step 84 in FIGS. 5-7, the current maintenance mode is stopped, the error counter is set to zero and a jump is performed to the mode "Pointer conditioning", arrow 86 in FIGS. 5-7. α is the desired degree of tolerance to the error in the multiple pointer list 56. In the mode "Pointer conditioning" the pointer error signal does not end in any measure, cf. FIG. 4.

At start of the system a correct pointer set must be made available to the queue system as soon as possible. Therefore the procedure "Pointer conditioning" is initiated. This can be performed either as a result of a pointer error appearing in accordance with the above, or by activation by means of an external signal. By pointer error is here and further on intended, as has been described above, that the number of copies of a pointer returned during the initiation procedure does not agree with the number stated in the multiple pointer list for the pointer in question. As soon as a pointer has been checked it is introduced into the idle list 38 and can be used for queueing packets.

When all pointers have been conditioned the next maintenance level 72 is executed for all pointers, below also denominated "Quick test". Thereafter the maintenance function 52 passes into the maintenance level 74, below also denominated "Fulltest", whereupon execution is started of the maintenance level 76 "Fulltest with retest" which is the current operational state, which however can be interrupted if a pointer error occurs.

If several erroneous pointers (multiple or lost pointers) than α are detected, the maintenance funtion 52 will always, as has likewise been described above, restart with "Pointer conditioning", independent of the one of the other maintenance functions that is executed. This is indicated in FIG. 3, as well as in FIGS. 5-7, by arrows 86.

With reference to FIG. 4, the mode 70 "Pointer conditioning" returns in step 88 each pointer x to the idle list 38, resets the value in the multiple pointer list 56 to "no multiple copies", and in the blocking list 62 to "no blocking". When the last pointer has been treated, cf. step 90, the pointer error counter is set to zero, step 92. The mode 70 thereupon ends in the mode 72, arrow 94.

The mode 72 "Quick test" is thus activated, with reference to FIG. 5, by the first mode 70 having been run through for all pointers. The initiation procedure is thereupon performed for all pointers and a simple write and read test on-line, step 96, is performed for buffer memory positions B(x) which have been identified by the pointer x. If found necessary, this test can be performed after queueing with low or lowest priority amongst the operations which can be performed on the buffer memory.

The result of the write and read test is used so as to return the current pointer to the idle list 38 if the test has given a correct result, but to block it from use in the system and introduce it into the blocking list 62 if the test indicates malfunction within the area identified by the pointer in question. After the last pointer the test passes into the mode 74, cf. step 98 and arrow 100.

In contrast to the off-line test described earlier above, wherein the test logic 14 in FIG. 1 controls and checks all data and all addresses to the memory, the test logic 58 in FIG. 2 at the on-line test has only write and read rights to the address positions which have been released from system use by the initiation procedure, wherein the system is emptied of a pointer value. With respect to everything described henceforth, the test logic is forced to operate only when no higher priority system operations towards the memory come into question.

In the simple write and read test in the mode 72 according to FIG. 5 a first bit pattern is written into the address positions identified by the released pointer. As soon as this has been carried through these positions are read for testing that the memory cells are able to store this first bit pattern. Thereupon the course is repeated a second time, this time with a second bit pattern being the inverse of the first pattern. If read data do not correspond to the write data the signal 60" in FIG. 2 is generated in the form of an error signal.

The third operational mode 74 "Full test" is activated, referring to FIG. 6, by the second mode 72 having been gone through for all pointers and starts by performing the initiation procedure anew on the pointers not blocked by the mode 72. Thereupon a buffer memory test is queued for detecting types of malfunctions, which have not been detected by the simple write and read test in the second operational mode, with lowest priority to be performed on the buffer memory positions B(x) being non-blocked, cf. steps 102 and 104. These memory positions are exercised, i.e. written and read, with test data, whereas all other buffer memory positions being non-blocked are exercised, i.e. written and read, with system data during the test for each blocked pointer.

The result of the test is used so as to return the pointer to the idle list 38 if the buffer memory test has given a correct result, but to block it from being used in the system, and introduce it into the blocking list 62 if the buffer memory test indicates malfunction of the area identified by the pointer. After the last pointer the test proceeds to the mode 76, cf. step 106 and arrow 108.

In case of complete on-line test according to the third operational mode 74 also errors in the addressing logic shall be detected. Normally this requires that data different from the data in the memory position B(x) is written into all other addresses. According to the invention, system data is written into and read out from the other non-blocked positions. This fulfills the same function as the controlled data in the earlier described off-line test. The risk that an addressing error will escape discovery by the system happening to write the same data as the test logic uses for the test, in an address having an address decoding error overlapping memory positions under test, is minimized by continously repeating all tests, and by one error detection being enough for blocking the pointer. Therefore the test logic awaits that system data have been written into all other used positions before the read check is performed.

The fourth operational mode 76 "Fulltest with retest" is thus activated, with reference to FIG. 7, by the fact that the third mode 74 has been run through for all pointers. Also the earlier blocked pointers are accepted thereupon for testing by said initiation procedure being performed cyclically for all pointers, one at a time. Thereupon a buffer memory test of the same kind as in the third mode is applied to non-blocked pointers, steps 110 and 112. Earlier blocked pointers are subjected to the same type of buffer memory test conditioned by each testing data operation being maintained in the queue as long as there is system data stored anywhere in the buffer memory segment containing the storing area B(x), which is identified by the earlier blocked pointer x, step 114. The result of the test is used in the same way as in the third mode for non-blocked pointers and not allowed tests of earlier blocked pointers, whereas earlier blocked pointers are returned to the idle list 38 not until having been accepted in a number of consecutive tests, said number being defined by a limit value β.

The fourth operational mode 76 is based on the understanding that in a continous operation the risk must also be taken into consideration that addressing errors from the memory position under test may corrupt system data. Therefore no writing is allowed from the test logic in earlier blocked positions before the whole segment in which addressing errors may occur, has been emptied of system data. Thereafter writing with system data in all other positions is awaited, as in the third operational mode. Since all read operations may corrupt data in erroneous memories, emptying of system data from the memory segment is anew awaited before read check is performed on the buffer positions which are identified by the earlier blocked pointer, which is now under anewed test.

In one embodiment of the case one single operational mode, there is essentially the question of a merger of the two last operational modes according to FIGS. 6 and 7.

More particularly, a test of the buffer memory 32 is queued after the initiation procedure, with the lowest priority, for being performed on the buffer memory positions which are identified by non-blocked pointers, cf. steps 102 and 104 in FIG. 6. These memory positions are exercised with test data. All other non-blocked buffer memory positions are exercised with system data during the testing procedure. Thereafter the result of the test is used in a way that the pointer is returned to the idle list 38 if the buffer memory test has given a correct result, but is blocked from use in the system if the buffer memory test indicates malfunction in the area defined by the pointer.

Blocked pointers are supplied to the blocking list 62 and are subjected to the same type of buffer memory test conditioned by each testing data operation being maintained in the queue as long as system data are stored anywhere in the buffer memory segment containing the positions identified by blocked pointers, cf. step 114 in FIG. 7. Thereafter the result of the test is used so as to return the pointer to the idle list 38 if the buffer memory test has given a correct result. Blocked pointers are returned to the idle list 38 first after having been accepted in a number of consecutive tests, greater than the limit value β.